This invention relates in general to fuel injectors for an internal combustion engine and more specifically to an electronic fuel injector in which the timing chamber is filled with fuel before filling the metering chamber.
Electronic fuel injectors are frequently used in today's internal combustion engines. The electronic fuel injector provides precise and reliable fuel delivery into the cylinder of compression ignition and spark ignition engines. The precision and reliability of the electronic fuel injector has contributed to the goals of fuel efficiency, maximum practicable power output and control of undesired products of combustion. These and other benefits of electronic fuel injection systems are well-known and appropriately used to beneficial effect and the design of modern internal combustion engines.
In recent years, electronically controlled fuel injectors have found applications in the heavy duty engine market and, more particularly, the diesel engine market. An example of a system utilizing electronic fuel injectors is the CELECT Engine Control System manufactured by Cummins Engine of Columbus, Indiana and available with their N-14 engine, as well as other engine models. In this system, an electronic fuel injector, such as the injector 5 shown in FIG. 1 for example, is used to implement various CELECT fueling strategies. As is known in the art, the injector body 10 is connected to a nozzle assembly 22 via a nozzle retainer 36. A timing chamber 26 is defined by a portion of the central cylindrical bore 14, the lower exposed surface of the timing plunger 16 and the upper exposed face of the metering piston 17. A metering barrel 34 is located between the interior portions of the injector body 10 and nozzle assembly 22. A metering chamber 33 is defined by a cylindrical bore 15 of the metering barrel 34, the lower exposed surface of the metering plunger 17 and the upper exposed surface of a nozzle spacer 23. The timing plunger 16 protrudes into the base of a central cylindrical bore 18 but is not mechanically coupled to the coupling member 20. The coupling member 20 abuts the timing plunger 16 such that only a compressive load may be transferred from the coupling member 20 to the timing plunger 16.
The coupling member 20 is equipped with an annular stop 65, located at the bottom end of the coupling member 20. The stop 65 limits the translation of the coupling member 20 in the direction of the injection stroke. Extending further radially outward on a flange 72 of the coupling member 20 is a spring seat 66, through which a return spring 68 acts upon the coupling member 20 biasing it upward in the direction of the retraction stroke. The opposite end of the return spring 68 acts upon a spring seat 70, located on the injector body 10 at the base of a collar 74.
At the exposed end of the coupling member 20, pocket 76 and a bearing surface 80 are formed, upon which a link 78 acts to force coupling member 20 against the force created by the return spring 68 during the injection stroke. The link 78 is typically in direct or indirect contact with the injection train cam shaft (not shown) and reciprocates along the central axis of injector assembly 5 in response to the angular rotation of the actuating cam (not shown). Thus, rotational motion of the cam shaft is converted into reciprocal motion of the injector assembly 5 axial components so as to provide force useful in pressurizing the timing chamber 26 and, ultimately, the metering chamber 33.
The fuel inlet port 45 is in communication with two separate fuel inlet branches. The first branch communicates the port 45 to the metering chamber 33 through a metering inlet 49 and check valve 35. The second branch communicates the port 45 to a control chamber 54, and ultimately the timing chamber 26, through a control inlet passage 47. Fuel flow from the control chamber 54 to the timing chamber 26 is accomplished by allowing the fuel to flow through the control valve 56, a control passage 50, a plunger chamber control orifice 48, and a plunger chamber passage 46 formed by an annular gap between the timing plunger 16 and the central cylindrical bore 14.
The basic operation of the injector is well-known in the art. A predetermined quantity of fuel is metered into injector assembly 5 during a retraction stroke and injected into the engine during an injection stroke. Fuel metering is controlled by the movement of the timing plunger 16, the metering piston 17, and the opening of a control valve 56 of the control solenoid 58. At the start of the retraction stroke (as shown in FIG. 1), the timing plunger 16 is substantially bottomed against the metering piston 17, the metering piston 17 is bottomed against the nozzle spacer 23 and the control valve 56 is closed.
As the fuel enters the injector body 10, fuel at rail pressure of 150 psi passes through the inlet passage 49 and opens the check valve 35 and enters the then very small volume of the metering chamber 33. The pressure of the fuel acting on the bottom of the metering piston 17 within metering chamber 33 forces metering piston 17 upward, thus creating additional pressure in timing chamber 26. As the cam profile allows the link 78 and the coupling member 20 to move upward under the urging of the spring 68 the pressure in timing plunger chamber 26 acts on the bottom surface area of timing plunger 16 thereby causing both the timing plunger 16 and metering piston 17 to independently move upward, with timing plunger 16 maintaining contact with coupling member 20. Fuel continues to flow through the check valve 35 into the expanding volume of metering chamber 33 as long as the control valve 56 is closed, which prevents fuel flow through the passage 50, the orifice 48 and the passage 46 into the collapsed timing chamber 26. When the control solenoid 58 is actuated by well-known means, the control valve 56 is commanded open and the metering of fuel into metering plunger chamber 33 ceases. This is accomplished by supplying fuel, also at rail pressure of 150 psi, from the control chamber 54, through the control valve 56, the passage 50 and the orifice 48, and the passage 46 into the timing chamber 26, thereby causing equal pressures to exist in both the timing chamber 26 and the metering chamber 33. Equal pressures acting on both ends of the metering piston 17 tends to stop its upward motion. Thus, a fixed and predetermined amount of fuel will remain in the metering chamber 33.
A bias spring 55, located within the timing chamber 26 and bearing against the opposing surfaces of the timing plunger 16 and the metering piston 17, ensures that the metering piston 17 remains stationary and does not drift up as the timing chamber 26 fills with fuel thereby continuing to force the timing plunger 16 upward. At the beginning of the retraction stroke, when the timing plunger 16 is bottomed against the metering piston 17, the spring 55 exerts a bias on these opposing surfaces of approximately 40 psi. As the timing chamber 26 fills with fuel, thereby causing the timing plunger 16 to move away from the metering piston 17, the biasing force of spring 55 decreases linearly. When the timing plunger 16 is maximally displaced from the metering piston 17 at the end of the retraction stroke, the bias of spring 55 is approximately 20 psi. Thus, at the end of the retraction stroke, the bias of spring 55 reduces to approximately 50% of its bias value at the beginning of the retraction stroke. The spring 55 also exerts enough force on the check valve 35, through the metering piston 17 and the hydraulic link created by the fuel located in the metering chamber 33, to keep the check valve 35 seated, preventing any change in the volume of fuel contained in the metering chamber 33. Thus, a precisely metered quantity of fuel is trapped in the metering chamber 33. This fuel is the quantity of fuel that will be injected into the engine during the subsequent injection stroke. As the retraction stroke continues, the timing plunger 16 continues to rise and the timing plunger chamber 26 continues to be filled with fuel at rail pressure until the end of the retraction stroke.
Details of the injection stroke including timing spill 106, start of injection 107 and end of injection 108, are not germane to the present invention, although a complete explanation may be found in U.S. Pat. No. 5,067,464, issued Nov. 26, 1991 to Rix, et al. herein incorporated by reference.
Referring to FIG. 2, a timing diagram of this well-known fuel strategy is shown. Under normal operation, the control valve 56 is closed at the beginning of the retraction stroke 110, thereby inhibiting the passage of fuel to the timing chamber 26. The fuel rail pressure of 150 psi is more than adequate to overcome the bias on spring 55 so that fuel enters the metering chamber 33 and displaces both the metering piston 17 and the timing plunger 16 away from the metering chamber 33. This initiates the start of metering 102. When a predetermined quantity of fuel has entered the metering chamber 33, the control valve 56 opens and permits fuel, at rail pressure of 150 psi, to enter the timing chamber 26. Since the pressure in the timing chamber 26 is now equal to the pressure in the metering chamber 33, the upward motion of the metering piston 17 ceases, thereby trapping in the metering chamber 33, the predetermined quantity of fuel to be injected into the engine during the injection stroke. This opening of the control valve 56 thus signals the end of metering 103. As the injector assembly 5 continues through its retraction stroke, the timing plunger 16 is forced by incoming fuel to continue moving away from the metering chamber 33. This portion of the retraction stroke is known as timing fill 105. At the end of the retraction stroke 106, the metering chamber 33 holds a predetermined amount of fuel to be injected into the engine and the timing chamber 26 holds a quantity of fuel defined by the top of the metering piston 17 and the bottom of the timing plunger 16.
A problem with the CELECT fuel system is known to occur during cranking at start up and at low rpm operation. When the N-14 engine operates at less than 1000 rpm, the fuel pressure may be less than 40 psi due to internal leakage. This may be inadequate to overcome the bias of spring 55 at the beginning of the retraction stroke and could therefore result in inadequate fueling during start up and operation below 1000 rpm. To address this problem, the N-14 uses a large fuel pump (1.25" PTG-based) and the CELECT strategy calls for full fueling (350 mm.sup.3 /stroke). However, this strategy results in uncontrolled smoke and emissions during start up. Preliminary estimates indicate that this uncontrolled start up mode accounts for approximately 12% of particulate emissions as measured on the EPA cycle. Moreover, estimates of fueling required for N-14 start up are in the range of 100 mm.sup.3 /stroke. Thus the actual fueling strategy overfuels the engine during cranking and start up and at engine speeds of less than 1000 rpm.
If metering could occur later during the retraction stroke, when the load on bias spring 55 is near 20 psi, controlled volumetric metering of fuel into metering chamber 33 could be accomplished. Such a fueling strategy would result in better smoke and particulate control during start up, since a precise quantity of fuel could be metered during the retraction stroke. Moreover, a smaller gear pump (0.75-1.00" PTG-based) may be adequate to supply fuel through the full operating range. A smaller and thus lower cost pump would be desirable if it could meet the cranking flow requirements. Finally, late metering may be advantageous in that it reduces the control delay between commanded fueling and combustion, thereby enabling a more precise control over speed, torque, emissions and smoke.